What We Do-- And Don't-- Understand About The Physics Of The Winter Olympics

Jessica Diggins, left, of the United States, celebrates after winning the gold medal past Stina Nilsson, of Sweden, in the during women's team sprint freestyle cross-country skiing final at the 2018 Winter Olympics in Pyeongchang, South Korea, Wednesday, Feb. 21, 2018. (AP Photo/Dmitri Lovetsky)

The 2018 winter Olympics in Pyeongchang are winding down, which feels like a bit of a missed opportunity for me. In the same way that the Winter Olympics represent a once-every-four-years opportunity for millions of people to pretend they understand how curling works, the Olympics are always a great chance to talk about physics. Alas, I've been much too busy to do as much of that as I'd like-- I did write one article about luge, but then work and family travel got in the way.

It's a shame, because the enormous amount of media coverage generated by the Olympics produces images of unparalleled quality to illustrate those sports. The New York Times just released another set of breathtaking composite images of Olympians in competition. These provide striking visual evidence of just how complicated these sports are, and how small a gap separates a gold medallist from the rest of the field.

You can also expect these images to be projected in the front of many a physics classroom over the next weeks and months, because on the level of things we can photograph, we understand the physics involved very well, and can use the images to illustrate the principles involved. And pretty much any sport at the Olympics has some physics content.

Kim Kyeongae, of South Korea, throws during their women's curling final in the Gangneung Curling Centre at the 2018 Winter Olympics in Gangneung, South Korea, Sunday, Feb. 25, 2018. (AP Photo/Aaron Favila)

The sport that probably most vividly conjures images of high-school physics is curling, which is experiencing a sudden surge in popularity thanks to the unexpected gold medal for the US men's team. The core activity looks a lot like a common physics demonstration: a rock sliding along at some speed strikes another rock at rest, which moves off at some speed and angle. It's a basic two-dimensional collision, the sort of thing that haunts the dreams of both first-semester physics students and the faculty who have to grade homework sets from first-semester physics.

If it were just banging rocks together, though, curling wouldn't be a team sport. What makes it Olympic-worthy is physics, mostly having to do with friction. All that frantic sweeping activity (which, by the way, is really hard-- you have to put a lot of pressure on those brooms to make a difference) is done to control the friction of the ice in front of the sliding stone. By selectively sweeping some parts and not others, and putting a little spin on the stone when it's released, curling teams can bend their shots around obstacles to an impressive degree.

Or take the case of skiing, which is hugely influenced by both friction and another force from the same annoying-to-a-physicist category, air resistance. When you see action photos of top skiers in action, they're always bent over in a tight tuck position, to minimize the amount of area they expose to the wind. This reduces the drag force that they experience, allowing them to accelerate more rapidly down the hill.

Victory in skiing is not as simple as just tucking into a ball and going straight, though: it requires steering a path down a course with numerous turns. The skier who wins is generally the one who does the best job of steering through the course, hugging the inside of the turns to minimize the total distance they have to cover. This helps in two ways: the shorter distance reduces the time needed to cover it at a given speed, and reducing the distance also reduces the effect of friction. The feature that puts friction in the annoying-to-a-physicist category is precisely that its effect depends on the path you follow. The longer the path, the more energy a skier loses to friction, and the slower they'll be moving along that path.

And, of course, there's the sport that traditionally dominates Winter Olympics coverage in the US, figure skating. This is a favorite of physics professors because it illustrates one of the most important principle in physics, the conservation of angular momentum.

A skater heading into a spinning jump will generally start with arms spread wide, and then draw their arms tight during the jump, throwing them wide again as they land. This isn't merely for dramatic flair, it's dictated by physics. As they start their initial rotation with arms wide, they have some amount of angular momentum, determined by the rate of spin and their distribution of mass. When they draw their arms in, they're moving mass closer to the axis of the spin, which would give them a smaller angular momentum. The laws of physics, though, tell us that in the absence of some other force, the angular momentum of a spinning object must remain constant, so their rate of spin increases to compensate for the re-arrangement of their mass. As they land after an astonishing number of rapid rotations, they throw their arms wide again, reducing the rate of spin again, and allowing them to skate on straight.

The same principle applies to the aerial tricks done by skiers and snowboarders-- more complicated tricks almost always involve the athlete starting out making themselves as tall or wide as they can, then drawing in to make themselves spin faster, then spreading out to regain control. Pretty much any sport that involves spinning and jumping will show this kind of pattern, and turn up in introductory physics classes.

Chloe Kim, of the United States, jumps during the women's halfpipe finals at Phoenix Snow Park at the 2018 Winter Olympics in Pyeongchang, South Korea, Tuesday, Feb. 13, 2018. (AP Photo/Kin Cheung)

While I spend a fair amount of time teaching intro physics, though, my professional background is in dealing with physics at a far smaller scale, that of atoms and molecules. And this, to me, is the most fascinating part of the Winter Olympics, because it's the one bit of Olympic physics that even in 2018 is not perfectly understood.

The one feature all of these sports have in common is that they all depend on the slipperiness of ice and snow. Curling stones slide down the rink because ice is slippery, top skiers attain high enough speed for air resistance to matter because packed snow is slippery, and figure skaters glide into turns and leaps with such grace because ice is slippery. And why ice is slippery is a surprisingly difficult question to answer-- I wrote about this here a couple of years ago, and the state of the science really hasn't changed, as illustrated by this Vox article by Brian Resnick.

The key to the slipperiness of ice is a thin liquid layer at the surface, and the source of this has long been debated in physics-- for many years, the standard explanation was that pressure from above lowered the melting point beneath a skate blade, but that effect really isn't big enough to explain why skaters can skate in extreme cold, let alone why ice beneath your boots, which have a large area and thus generate little pressure, should be slippery. The modern explanation is that the thin liquid layer is just a general feature of solid surfaces that are fairly close to their melting point. When the regular crystal structure found deep in the ice stops at the boundary, there's a general disordering of the bonds between molecules that makes molecules right at the surface more liquid than solid, and creates the slipperiness that we experience.

Children sled down a hill on a golf course at the Isle of Palms, S.C., Wednesday, Jan. 3, 2018. (AP Photo/Mic Smith)

This is a wonderful example of a "More Is Different" phenomenon, to lift the title of a famous article by Nobel laureate Philip Anderson. There's nothing obvious about the properties of individual water molecules to tell you that ice should be slippery; instead, it's a phenomenon that only emerges when you freeze vast numbers of molecules into a solid. And the presence of the boundary is absolutely essential-- it's not enough to have a bulk solid, you need a surface where the solid stops and the air begins.

This is an enormously complicated problem to attack from first principles: if you want to start with individual water molecules, and end up showing that ice is slippery, you better have a pretty big supercomputer to run your simulations on. And yet, the slipperiness of ice and snow is something that even a small kid can experience, and enjoy with a plastic sled. As a physicist, I find that as wonderful as any of the athletic feats we've seen from Pyeongchang over the last couple of weeks.

So, anyway, we bid a fond farewell to the 2018 Winter Olympics. It's been a great and highly entertaining showcase, both for the potential of exceptionally dedicated humans, and the universal laws of physics.